In the past, various attempts have been made to improve the cutting capabilities of conventional twist drills. While some of these improvements have been accepted and incorporated into drills for use with conventional metals, exotic materials such as titanium, Inconel, Waspaloy, Rene' 41 and 95, have still proven to be extremely difficult to drill, especially if one is after the capability of drilling a large number of holes between re-sharpenings.
These metals place conflicting demands upon the drill. First, there is a need for high-torsional strength to prevent the tendency of the drill to "unwind" which causes chatter and damage to the cutting edges. Second, there is a requirement for sufficient tool body at the cutting edge to dissipate the heat generated during drilling. Conventional general purpose drills have helix angles of approximately 30.degree., thereby providing substantial tool body at the cutting edges; but they do not have adequate torsional strength and, therefore, are prone to chattering. Greatly increasing the helix angle, to 45.degree., for example, reduces the chattering, but it also reduces the tool body at the cutting edge. Hence, cutting-edge strength and the ability to dissipate heat are also lost. Additionally, when helix angles are increased, chip flow is impaired, causing the flutes to clog, particularly when drilling metals such as aluminum, thereby nullifying the benefits of increased torsional strength.
National Aerospace Standard 907 (hereafter abbreviated NAS 907) sets forth present day drill design philosophy for these difficult-to-drill, high-strength metals. But drills meeting the NAS 907 dimensional standards still do not provide sufficient drill life. It has been determined that even small dimensional changes, or just the tightening of tolerances on a given dimension, can radically change the effectiveness of a drill. For example, in U.S. Pat. No. 3,387,511, "Twist Drill" by W. K. Ackart, Sr, et al. (Applicant is co-inventor), slight changes in the chisel-edge angle, rake angle, and secondary section to chisel-edge angle obtained a significant increase in drill life over then existing drills.
Additional patents of interest include U.S. Pat. No. 4,116,580, "All Cutting Edge Drill" by R. F. Hall, et al.; U.S. Pat. No. 2,936,658, "Twist Drill" by O. L Riley; U.S. Pat. Nos. 4,222,690, "Drill Having Cutting Edges with the Greatest Curvature at the Central Portion Thereof" by R. Hosoi, and 4,065,224, "Twist Drill" by K. Siddall. While the above list of patents are felt to be of interest, they are not considered to be relevant.
Therefore, it is a primary object of this invention to provide a twist drill having improved heat dissipation and chip-flow qualities.
Another object of the invention is to provide a twist drill having high-torsional strength and greater cutting edge strength.
A further object of the invention is to provide a twist drill with improved cutting geometry which will provide for a greater number of holes to be drilled between resharpenings than existing drills.